ERNST G. HUF 



225 



leading lo an apparent sodiimi delicil in the outside solution. This points to a 

 H'^^zi Na+ exchange between skin and outside bath. It has been calculated that 

 production of (X)o and formation of HCO:r by skin could account for the ap- 

 parent sodium det'icit (24). 



Xext to sodium, potassium has been given more careful attention (tig. 11). 

 It appears that for physiological levels of extracellular sodium and potassium, 

 and also for most conditions of lowered or elevated sodium and potassium 

 levels, {X)tassium is released from and rejected by the epithelium. Low extra- 

 cellular potassium of the bath leads also to loss of potassium from the corium. 

 Inside leakage, however, decreases with increasing extracellular potassium and 

 from a certain potassium level on, which varies with the extracellular sodium 

 level, potassium is actually taken up at the corium side of the skin, is then 

 transferred across the skin and finally released into the outside bath (fig. 12). 



Potassium ion movements in isolated skin have also been studied with rela- 



Table 2. Ion distribution across frog skin 



12 hours after the beginning of 12 experiments, in which 0.4 Ringer's was placed at both 

 sides of the skin. Method of paired bags. Average data. Volume reduction at epithelial side 

 in % of original amount present there. Ion concentrations in ;uEq/ml. Indices o and i refer 

 to outside and inside compartment, respective!}-. Outside compartment, 5 ml; inside com- 

 partment, 2.5 1. 



tion to skin potentials. Levi and Ussing (43) found that, at about 50 mv, influx 

 and outflux of tracer potassium are equal. Similar results w^ere obtained when 

 conventional chemical methods were applied in studies on net rates of transport 

 of potassium (28). No net potassium accumulation in the outside bath takes 

 place at potentials lower than 40 mv. This suggests that, at potentials lower 

 than 40 mv, small amounts of potassium are actively taken up by the epithelium 

 and moved, together with sodium chloride, into the skin. More often, however, 

 potassium seems to behave as a passively moving ion that is driven out of the 

 epithelium under the influence of strong electrostatic forces. A sharp and 

 considerable increase in skin potential can be induced by adding some sodium 

 iodide to the outside bath (27). As a result of this, net potassium accumulation 

 in the outside bath increases by several 100% (27). Potassium accumulation at 

 the epithelial side of the skin seems to be largely a function of skin potential 

 (28; see fig. 13). Some revision or extension of this interpretation would seem 

 necessary, however, since it was also found that potassium is released from the 

 corium under conditions of low extracellular potassium levels. The difficulty that 



